<!DOCTYPE html>
<html>
<head>
  <meta charset="UTF-8">
  <title>Document</title>
</head>
<body>
  <script>
    function countDown(time) {
      var nowTime = new Date();
      var overTime = new Date(time);
      var times = (overTime - nowTime) / 1000;
      var d = parseInt(times / 60 / 60 / 24);       // 天数
      d = d < 10 ? '0' + d : d;
      var h = parseInt(times / 60 / 60 % 24);       // 小时
      h = h < 10 ? '0' + h : h;
      var m = parseInt(times / 60 % 60);            // 分钟
      m = m < 10 ? '0' + m : m;
      var s = parseInt(times % 60);                 // 秒数
      s = s < 10 ? '0' + s : s;
      return d + '天' + h + '时' + m + '分' + s + '秒';
    }
    console.log(countDown('2021-9-25 09:00:00'));
  </script>
</body>
</html>